Simple kinetic energy transfers
Simple demonstrations to introduce the concept of kinetic (motion) energy.
Apparatus and materials
Pulley, single, on clamp
Mass hanger and slotted masses (100 g)
G-clamp, 10 cm
Health & Safety and Technical notes
In all of these experiments a student should act as a 'trolley catcher' to ensure no trolleys land on toes.
a Put the trolley on a bench and give it a push. Chemical (food + oxygen) energy stored in muscles is transferred to kinetic (motion) energy.
Instead of a trolley you could, more impressively, use a student on a skate board (with due consideration to student behaviour and safety).
b Put a trolley on the bench. Fasten the pulley to the edge of the bench, running a thread over it from the trolley to a 100 g hanger. Let the load fall a short distance to the floor so that the thread falls slack, allowing the trolley to continue moving. Gravitational potential energy is transferred to kinetic (motion) energy.
Repeat the demonstration, but in reverse. Start the trolley moving with a push away from the pulley. Let it pull the thread taut and lift the load as it comes to rest. Kinetic energy is transferred to gravitational potential energy.
c Before you use the expendable spring, stretch it until it is clearly an open, weak spring. Anchor one end (A) to the bench by slipping the end loop over the rod of a retort stand, which is itself clamped to the bench.
Fasten 1 – 2 m of thread to the other end (B) of the spring. Then stretch the spring gently by at least several centimetres. Anchor this end (B) temporarily.
Straighten out the thread and attach the free end to a trolley. Position the trolley so that the thread is taut and then release the end B. You need to have a long enough piece of thread for the trolley to travel 30 cm or so at constant speed after the spring has fully contracted.
Chemical energy in muscles is transferred to elastic (strain) energy of the spring, which is then transferred to kinetic (motion) energy.
Repeat the experiment, but in reverse. Give the trolley a push so that it causes the spring to stretch. Kinetic (motion) energy is transferred to elastic energy. If you make any compensation for friction, by tilting the board, this will have to be adjusted when you reverse the transfers.
1 The concept of kinetic (motion) energy is very important. It describes how energy stored in fuels can be transferred to rockets, gas molecules and anything else which is moving.
Before students can tackle calculations with kinetic energy, they need a clear picture of what kinetic energy is, and a good feeling for it. These simple qualitative demonstrations invite discussion. It is useful to use them before attempts to measure and calculate values for kinetic energy.
This experiment was safety-checked in November 2005